Fastener assemblies comprising metal screws and sealing rings, such as O-rings made from synthetic rubber, are used widely in various sealing applications. In one widespread application, such fastener assemblies are used as drain plugs for oil pans of internal combustion engines, as for motor vehicles.
In one known arrangement, the screw head has an undercut, annular recess to accommodate an O-ring, which is intended to form a seal between the screw head and a workpiece when a threaded portion of the screw shank is driven into a threaded aperture in the workpiece. The aperture may have a countersink. See, e.g., Crull U.S. Pat. No. 4,701,088.
Problems are encountered with the known arrangement noted in the preceding paragraph and are attributable to cumulative, dimensional tolerances associated with the O-ring dimensions, the recess dimensions, and any countersink dimensions. One such problem is that, if the O-ring material has excessive volume relative to the recess and any countersink, the screw head cannot achieve metal-to-metal contact with the workpiece and can bear only against the O-ring material. Eventually, the O-ring material exhibits cold flow, which results in a loosened seal. Another such problem is that, if the O-ring material has insufficient volume relative to the recess and any countersink, an effective seal is not achievable. These two problems present particular difficulties when the fastener assembly is used as a drain plug for an oil pan of an internal combustion engine, as for an automobile or a truck, in which oil leaks can lead to severe engine damage.
There has been a need, to which this invention is addressed, for improvements averting both of these problems in such a fastener assembly.